Different bases for different correlation effects: multireference Møller–Plesset perturbation theory in the extended basis function space

A method to improve virtual orbitals of CASSCF theory has been proposed by expanding the virtual orbital space through the addition of basis functions without disturbing already established CASSCF function space. It is a natural extension of work used in Hartree–Fock calculations. The improved virtual orbitals are then used to estimate the dynamical correlation. That is, a moderate basis set is used in CASSCF to include nondynamical correlation and a sufficiently large basis set is used in multireference Møller–Plesset perturbation theory to estimate the slowly convergent dynamical correlation. The present formalism is tested on the barrier height of the H2CO → H2+CO reaction, singlet–triplet energy separation in CH2 and valence π–π* excitation energies in butadiene. Numerical results show that good accuracy can be obtained.